Aluminum and aluminum alloy metals typically need to be coated otherwise they rust or display other undesirable effects from exposure to atmosphere and moisture.
Chromating has been the method of choice in aluminum finishing and aerospace industry for pretreating of all types of aluminum alloys for many decades. The chromate conversion coating formed on aluminum surface serves two basic purposes: stand-alone temporary protection of the metal against corrosion and as a base for adhesion to paint overcoat. The former is achieved via electrochemical and barrier passivation of aluminum by an Al2O3—Cr2O3 mixed oxide layer, and the latter owes, in large part, to increased surface area of chromated surface. The chromating reaction proceeds by Cr(VI) oxidizing the Al to Al (III), which forms an amorphous mixed oxide layer. This reaction is by no means stoichiometric, and excess Cr(VI) are often present in the resultant oxide film. When fresh metal surface is exposed as a result of physical impact and at suitable humidity levels, the remnant Cr(VI) in the film can slowly leach out to oxidize and seal the “wound”, a phenomenon known as “self-healing”. Nonetheless, use of chromates is under ever-tightening regulations because chromate has been identified as a human carcinogen.
Exploring health/environment-benign alternatives to chromates with comparable corrosion protection performance has been underway for well over a decade. To date, the new types of chemistry investigated have only partially met the goal. Those proposed new conversion coatings often necessitate using other transition metals (although less toxic), and/or fail to meet the same performance standard set by chromates in terms of both stand-alone protection and paint adhesion.
Among different conversion coating systems examined, silane-based one possesses several valuable characteristics. Silane based coatings are completely metal-free (therefore truly “green”), and they can covalently bind the paint to the metal, leading to superior paint adhesion. Organofunctional silanes have long been used as coupling agents for binding two surfaces of different chemistries, such as fiberglass to plastics and rubber to metals. Commonly referred to as “organic-inorganic hybrid” compounds, organofunctional silanes have reactive organic functional groups on one end (such as epoxy, amino, acryl, etc.) and hydrolysable alkoxysilyl groups on the other. Coupling to paint resins is effected via reaction between the organic functional groups of the silane and those of resin molecules; while coupling to metal surfaces occurs via formation of metal-oxygen-silicon, or M—O—Si bonds, where M is equal to metal. When applied from water solution at acidic pH, the hydrophobic alkoxysilyl group of the silane hydrolyzes to hydrophilic silanol groups that are more compatible, in terms of surface energy, with that of hydrophilic metal oxide surfaces.
Investigation on using silane coupling agents as replacement to chromates has been pioneered by Van Ooij et al. Initial efforts of other research groups were largely confined to monofunctional silane, i.e., silane with one hydrolysable alkoxysilyl group. Monofunctional silane —X—R—Si—(OR′)3, where X is the organic functional group, tends to form a linear siloxane polymer with pendant silanol groups upon controlled hydrolysis. This might suggest that further condensation via those pendant silanols should give rise to a well crosslinked barrier film. However, it was found that the property of monofunctional silane-derived coating is hardly satisfactory without using additional crosslinkers such as tetraethoxysilane (TEOS) or tetravalent Zr, and the applicable life of the coating solution is very short. Problematic still, in diluted water solutions, the monofunctional silanes also tend to form a monolayer on hydroxylated or silaceous surfaces via M—Si—O bonds, leaving no —Si(OR′)3 groups available to crosslink with other silane molecules and unable to build up a thicker film that is essential to corrosion protection.
The Van Ooij group has determined that multifunctional silanes (silanes with more than one alkoxysilyl groups) are much effective at forming protection layer on aluminum. This finding underscores the importance of film-forming properties of the silanes when it comes to corrosion protection of unpainted metals. It is believed, without intending to be bound thereby, that corrosion of a coated metal surface involves diffusion of corrosive species from environment to the paint/metal interface, which can be hindered when the diffusion path is made tortuous and diffusivity reduced by high degree of crosslinking of the coating layer.
The situation is very different in the case of bifunctional silanes, denoted as (R′O)3—Si—R—Si—(OR′)3, where R is a bridging group with or without heteroatoms. This bifunctional silane is capable of covalently binding to the native metal oxide on metal surfaces through one of the two alkoxysilyl groups and of condensing/crosslinking among themselves through the other. The nature of thus crosslinked matrix is no longer that of a siloxane, but of an organic/inorganic hybrid material.
Bis-type silanes reported in literature for use in corrosion protection of metals include bis-(3-triethoxysilylpropyl)tetrasulfane(BTSPS), bis-1,2-[triethoxysilyl]ethane (BTSE), bis-1,2-[trimethoxylsilylpropyl]amine (BTSPA), all of which are commercially available. BTSE was the first bisfunctional silane explored and was soon discarded due to lack of reactive organic functional groups on the backbone ethylene group that is essential for paint adhesion. The sulfidesilane-BTSPS had been investigated as a protection layer on various grades of steel and aluminum alloys. A series of corrosion tests including neutral and copper accelerated salt spray, paint adhesion, hot salt immersion, as well as several electrochemical characterization demonstrated that overall performance of BTSPS is equivalent to, sometimes better than that of chromate conversion coating. It is believed that interaction between the sulfide —(S4)— and Fe atom significantly contributes to the electrochemical passivation of steel, and interaction between S4 group and topcoat functional residues enhances the adhesion of the silane layer to paints and rubbers. Although BTSPS also protects aluminum and zincated surfaces as well as it does to steel, it suffers two major drawbacks. It is solvent-borne and requires lengthy (often days of) hydrolysis prior to application due to its high hydrophobicity. Furthermore, although the bisamino silane-BTSPA is totally water soluble without any organic solvent, its corrosion protection performance of unpainted aluminum is far inferior to that of tetrasulfide silane. This can be partially explained by its lower hydrophobicity due to the presence of hydrophilic secondary amine group. Adding vinyltriacetoxylsilane-VTAS to BTSPA solution helps raise its performance to a certain degree, but the vinyl silane is not stable in water and is observed to slowly condense and precipitate out of solution over time.
The key to a successful silane-based metal pretreatment process lies in identifying a multifunctional silane with an ideal combination of water solubility (a practical issue), hydrophobicity (for best corrosion protection), high crosslinking capabilities (barrier to diffusion of corrosive species), slow rate of condensation (long solution life), and reactivity (for paint adhesion). However, neither tetrasulfide nor bisamino silane can meet all five requirements, nor can any current commercial silanes. Therefore, it would be desirable to identify new metal coatings that are useful to coat aluminum and aluminum alloys.